Solar cells that provide an environment-friendly electric energy source have been drawn public attentions as an effective energy source that can solve energy problems that have currently become more and more serious. At present, as a semiconductor material for use in photovoltaic devices for solar cells, inorganic substances, such as single crystal silicon, polycrystal silicon, amorphous silicon, and a compound semiconductor, have been used. However, since the solar cell to be produced by using inorganic semiconductors requires high costs in comparison with other power generation systems, such as thermal power generation and nucleic power generation, it has not been widely used for general household purposes. The main reason for the high costs lies in that a process for manufacturing a semiconductor thin-film under vacuum at high temperatures is required. For this reason, organic solar cells have been examined in which, as a semiconductor material that can desirably simplify the manufacturing process, an organic semiconductor and an organic colorant, such as a conjugated copolymer and an organic crystal, are utilized.
However, the largest problem with the organic solar cells using the conjugated polymer or the like is that its photoelectric conversion efficiency is low in comparison with conventional solar cells using inorganic semiconductors, and these solar cells have not been put into practical use. The reasons that the photoelectric conversion efficiency of the organic solar cells using the conjugated polymer is low mainly lie in that the absorbing efficiency of solar light is low, in that a bound state referred to as a bound exciton state in which electrons and holes generated by solar light are hardly separated is formed, and in that since a trap that captures carriers (electrons and holes) is easily formed, generated carriers are easily captured by the trap, with the result that the mobility of carriers is slow.
At present, the conventional photoelectric conversion device with the organic semiconductors can be generally classified into the following device structures: that is, a schotkky-type structure in which an electron donating organic material (p-type organic semiconductor) and metal having a small work function are joined to each other, and a hetero junction type structure in which an electron accepting organic material (n-type organic semiconductor) and an electron donating organic material (p-type organic semiconductor) are joined to each other. These devices have a low photoelectric conversion efficiency because only the organic layer (layer of about several molecules) of the joined portion is allowed to devote to photoelectric current generation, and the improvement thereof has been required.
As a method for improving the photoelectric conversion efficiency, a bulk hetero-junction type structure (for example, see J. J. M. Halls et al., “Nature,” No. 376, 1995, page 498) has been proposed in which an electron accepting organic material (n-type organic semiconductor) and an electron donating organic material (p-type organic semiconductor) are mixed with each other to increase the junction surface that devotes to the photoelectric conversion. In particular, a photoelectric conversion material has been reported (for example, see G. Yu et al., “Science,” Vol. 270, 1995, page 1789) in which a conjugated polymer is used as the electron donating organic material (p-type organic semiconductor) while a C60 derivative, such as PCBM, is used as the electron accepting organic material in addition to a conductive polymer having an n-type semiconductor characteristic.
Moreover, to effectively absorb radiating energy that covers a wide range of solar light spectra, another photoelectric conversion material using an organic semiconductor has been reported (for example, see E. Bundgaard et al., “Solar Energy Materials & Solar Cells,” Vol. 91, 2007, page 954) in which an electron donating group and an electron withdrawing group are introduced to a main chain so that a band gap is narrowed. Thiophene skeletons have been examined as this electron donating group, and benzothiazole skeletons and quinoxaline skeletons have been vigorously examined as this electron withdrawing group (for example, see E. Bundgaard et al.; A. Gadisa et al., “Advanced Functional Materials,” Vol. 17, 2007, pp 3836-3842; W. Mammo et al., “Solar Energy Materials & Solar Cells,” Vol. 91, 2007, pp 1010-1018; R. S. Ashraf et al., “Journal of Polymer Science Part A: Polymer Chemistry,” Vol. 44, 2006, pp 6952-6961; C-L. Liu et al., “Macromolecules,” Vol. 41, 2008, pp 6952-6959; N. Blouin et al., “Journal of American Chemical Society,” Vol. 130, 2008, pp 732-742; M. Sun et al., “Macromolecular Chemistry and Physics,” Vol. 208, 2007, pp 988-993; W-Y. Lee et al., “Macromolecular Chemistry and Physics,” Vol. 208, 2007, pp 1919-1927; A. Tsami et al., “Journal of Materials Chemistry,” Vol. 17, 2007, pp 1353-1355; and M. Lai et al., “Journal of Polymer Science Part A: Polymer Chemistry,” Vol. 47, 2009, pp 973-985, and Japanese Patent Application National Publication (Laid-Open) Nos. 2004-534863 and 2004-500464). However, these methods have failed to provide sufficient photoelectric conversion efficiency.
As described above, the conventional organic solar cells have a problem of a low photoelectric conversion efficiency. It could therefore be helpful to provide a photovoltaic device having a high photoelectric conversion efficiency.